Under the drive of Moore's Law, the feature size of the conventional MOSFET continues to shrink, now has to enter the nanometer scale, consequently, the negative effects such as short channel effect of a device and so on have become more serious. The effects of drain induced barrier lowering, band-to-band tunneling and so on cause a off-state leakage current of a device to increase continually, at the same time, a sub-threshold slope of the conventional MOSFET can not decrease synchronously with the shrink of the device size due to of the limitation by the thermal potential, thereby the device power consumption increases. Now the power consumption concern has become the most serious problem of limiting the device shrink by the same scale.
In order to enable the device to be applied in the field of ultra-low voltage and low power consumption, using a new turn-on mechanism to obtain a device structure with ultra-steep sub-threshold slope and a process and preparation method have become the focus of everyone's attention to small size devices. In recent years, researchers have proposed a possible solution of using tunneling field effect transistor (TFET). Different from the conventional MOSFET, TFET has source and drain with opposite doping types each other, can control the band-to-band tunneling of the reverse biased PIN junction by gate to achieve turn-on, can break through the limitation of the sub-threshold slope 60 mV/dec of the traditional MOSFET, and has a very small leakage current. TFET has many advantages such as low leakage current, low sub-threshold slope, low operating voltage and low power consumption, and so on. However, due to the limitation of source junction tunneling probability and tunneling area, TFET is faced with a problem of small on-state current, which is far less than the traditional MOSFET devices, and this greatly limits the applications of TFET device. In addition, it is difficult to achieve TFET device with a steep sub-threshold slope in the experiment, because it is more difficult to achieve a steep doping concentration gradient at the source junction in the experiment so that the electric field at the tunneling junction is not sufficiently large when the device turns on, which may cause a sub-threshold slope of TFET to degrade relative to the theoretical value. Therefore, it has become a further important issue in connection with TFET device how to achieve a steep doping concentration gradient at the source junction to obtain an ultra-low sub-threshold slope.